The STL import feature in FDTD Solutions, MODE Solutions, and DEVICE allows import of 3D structures from the STL CAD file format. However, there are a few restrictions to keep in mind when generating STL data for importing:
Only 1 object can be included in the STL file at a time. If multiple objects need to be imported, you can save them to separate STL files and import each object individually.
Surfaces of the structure can’t overlap. There is typically a merge function in the CAD software which will allow you to merge the faces of the structure to get rid of overlapping surfaces.
The surface which is defined must be a fully enclosed surface to form a solid object.
There are some additional tips on the KB page here: